Such a process is for example known from the patent application JP2000-233918A. This document describes a process wherein a hydrocarbon is first reformed into a syngas (generally used abbreviation for synthesis gas) mixture comprising CO, CO2, and H2 and optionally other components like water, hydrocarbons like methane, and nitrogen. The syngas is cooled, condensed water is separated, and then CO is separated with a pressure swing adsorption process (PSA). An off-gas stream comprising mainly CO2, H2 and some CO is heated to 740° C. in a heater, and then introduced into a RWGS reactor packed with a RWGS catalyst having Fe and Cr as its main components, to react part of the CO2 into CO.
The reverse water gas shift reaction, or the backward reaction generally referred to as water gas shift reaction, is an equilibrium reaction that can be represented as:CO2+H2⇄CO+H2O
Conversion of CO2 to CO by a catalytic RWGS reaction is recognized as a promising process for CO2 utilization, and has been subject of numerous publications. The equilibrium constant of the RWGS reaction would be about 1 at 800° C., meaning higher temperatures are needed to shift the equilibrium and favour CO being formed. Various catalysts have been proposed to enable lower reaction temperatures. Also depending on the catalyst applied, some methane may be formed as by-product as a result of methanation reactions:CO+3H2⇄CH4+H2OCO2+4H2⇄CH4+2H2O
In U.S. Pat. No. 5,714,657 a multistep process is described for converting natural gas into higher hydrocarbons via syngas, wherein the obtained gas mixture of CO and H2 is reacted with CO2 at a temperature of 800-1300° C. in the absence of a catalyst, to increase the amount of CO that is subsequently further reacted with water in a so-called Koelbel-Engelhard reaction. Process efficiency is increased by separating the by-product CO2 and using it in the thermal RWGS reaction.
A process wherein a hydrocarbon is reformed to give a syngas mixture, to which carbon dioxide is added and then contacted with a RWGS catalyst, is known from GB2168718A; but this publication provides no further details on reaction conditions or catalysts to be applied.
US2004/191164 relates to a process for the treatment of syngas to increase the content of H2 and/or CO by the reverse water gas shift reaction comprising the step of contacting the syngas with a manganese- and zirconium-oxide comprising catalyst. US2004/191164 describes that the inlet temperature of the zone in which the catalyst is confined must be between 500 and 1000° C. and that the used reactor allows for the feedgas to be heated to the desired temperature before reaching the catalyst.
US2003/113244 relates to a process for producing CO by reverse water gas shift reaction in the presence of zinc- and chromium-oxide catalyst, wherein the reaction is carried out at a temperature of 300-500° C. In the processes described in US2003/113244, the syngas mixture is heated to temperatures in the range of 300-520° C. before it is reacted by passing over a catalyst bed.
U.S. Pat. No. 5,911,964 relates to a method for reducing CO2 by catalytic reversed water gas shift reaction, which is characterized in that the catalyst comprises a transition metal on a zinc oxide-comprising carrier. The reaction is preferably conducted at a temperature of about 400-600° C. or more. U.S. Pat. No. 5,911,964 is silent on the temperature of the feed gas mixture before contacting it with the catalyst at reaction temperature.
U.S. Pat. No. 3,718,418 discloses a process wherein the reversed water gas shift reaction is carried out in the presence of Rh and Ru metals and alloys thereof with Pt as a catalyst. The process according to U.S. Pat. No. 3,718,418 is carried out by passing the feed gas into contact with the catalyst in a reactor chamber and heating said chamber within the range of 175-500° C. Preferably, the reactor chamber is maintained at a temperature of about 175-275° C.
US2007/0142482A1 discloses a method for producing dimethyl ether (DME) directly from syngas, wherein a CO2-rich stream is separated from a crude DME product stream and reacted with hydrogen in the presence of a RWGS catalyst at a temperature of 500-1000° C. to form a CO-rich stream, which is fed to the DME forming step. The amount of CO present in the CO2-rich stream is very low, e.g. well below 1 vol %.
A disadvantage of the known process for increasing the CO content of a feed gas mixture comprising CO2, H2 and CO via the RWGS reaction is that coke is being formed and deposited on e.g. reactor wall, gas-distribution plates or particles, and catalyst particles; leading to clogging of the reactor and/or catalyst deactivation.
Coke formation can be the result of various reactions; like thermal cracking of methane, or decomposition of carbon monoxide via the so-called Boudouard reaction:CO⇄C+CO2 
The Boudouard reaction is especially likely to occur in gas mixtures having a relatively high CO content.
It is therefore an objective of the present invention to provide a process for increasing the CO content of a feed gas mixture comprising CO2, H2 and CO via the RWGS reaction wherein no or very little coke formation occurs.